The present invention relates to industrial robots and more particularly to multiaxis electronic robot controllers having a capability for workpoint torque control.
In typical robot controllers, joint motor current and therefore workpoint torque are varied to satisfy robot position commands. Workpoint torque is accordingly varied as an intermediate variable to control workpoint position as an end controlled variable.
There are various robot applications where it is desirable or necessary that robot workpoint torque be controlled as end controlled variable. For example, because of possible small noncompliance between the robot reference axes and the external world reference axes, the actual workpoint position may need to be adjusted under direct torque control as opposed to position control to provide compliance with a position command and enable a task to be performed at the command position (such as inserting a bolt in a bolt hole at the command position).
As another example, it may be desirable to maintain torque along one world axis direction within a predefined range as robot arm endpoint motion occurs along another world axis. More generally, it is desirable that combined multiaxis workpoint torque and position control be provided for robot operations with arm dexterity control through the simultaneous control of robot arm endpoint position and torque as end controlled variables.
Some prior robot controllers have had the capability for providing control of workpoint torque as an end controlled variable. Since prior robot joint power amplifiers have typically been encased with the motor current loop control internalized within the power amplifier casing, workpoint torque control based on motor current feedback has generally not been possible.
Instead, force sensors mounted near the robot wrist have been used to generate feedback signals representing the physically sensed workpoint force. A higher level control loop in the robot controller in turn has processed the physical workpoint torque feedback signals to generate torque commands for the joint power amplifiers and thereby control the workpoint torque as an end controlled variable.
In general, the use of force sensors for workpoint torque control has provided accurate robot performance. However, combined robot workpoint torque and position control has generally not been possible and penalties have existed in the form of a higher pay load at the end of the robot arm and a relatively high cost especially for many robot applications classed as relatively simple.
With the development of a fully digital robot controller disclosed in the patent applications cross-referenced herein, joint motor current feedback signals are accessible for workpoint torque control. The present invention is directed to an improved robot controller in which workpoint torque is controlled as an end controlled variable as a function of motor current feedback.